Tubular heat exchanger

ABSTRACT

A tubular heat exchanger preferably of a multi-pass design, has a cylindrical shell with open ends that are releasable covered and sealed by preferably two end caps. Located removably in the shell and axially between the end caps is a core having a plurality of outer tubes and preferably a plurality of inner tubes with each one of the inner tubes extending through a respective one of the outer tubes. A plurality of perforated plates disposed in the shell are sealed releasably to respective ends of the inner and outer tubes thereby forming a plurality of liquid tight chambers for the flow of a plurality of segregated mediums and the transfer of heat therebetween.

FIELD OF THE INVENTION

The present invention relates to a tubular heat exchanger, and more particularly to a multi-pass, tubular, heat exchanger with a servicable core for marine applications.

BACKGROUND OF THE INVENTION

Tubular heat exchangers are known to flow two fluid mediums of different temperatures to transfer heat from one fluid to the other. Such heat exchangers generally have an outer shell that houses a central core having a plurality of tubes for transferring heat. The first ends of the tubes are supported by a perforated first plate sealed continuously to an inward face of the shell. The opposite second ends of the tubes are supported by a perforated second plate also sealed continuously to the inward face of the shell, thus defining a mid chamber between the plates for the flow of a first fluid medium. An inlet chamber for the flow of a second fluid is defined by the first plate and a first end of the shell, and an outlet chamber is defined by the second plate and an opposite second end of the shell. The second fluid flows from the inlet chamber, through the tubes and into the outlet chamber.

The first fluid flowing through the mid chamber generally envelopes the tubes for heat transfer through the tube walls and to the second fluid flowing through the tubes. For efficient heat transfer, the tubes are typically made of copper or a copper alloy. To maintain fluid segregation, each end of each tube must be reliably sealed to the respective first and second plates. Traditionally, this seal is created by an expensive brazing procedure between the copper tubes and the plates. Because of this brazing, the first and second plates must also be of a copper alloy.

During operation of the heat exchanger, thermal expansion and contraction is known to cause stress cracks between various brazed seals causing a loss of seal integrity. Moreover, known assembly methods limit the ability to feasibly manufacture a multi-pass, tubular, heat exchanger having multiple inlet and outlet chambers, thus the ability to control cooling or heating rates as a function of multiple fluid temperatures is limited. Yet further, known brazing techniques limit or prevent cost effective maintenance and replacement of individual parts of the heat exchanger core. Such ability is particularly needed where fluids tend to be corrosive, or in marine applications that use seawater as a coolant that is not only corrosive but may encourage marine growth and sediment build-up inside the heat exchanger.

SUMMARY OF THE INVENTION

A tubular heat exchanger preferably of a multi-pass design, has a substantially cylindrical shell with open ends that are releasable covered and sealed by end caps. Located removably in the shell and axially between the end caps is a core having a plurality of outer tubes and preferably a plurality of inner tubes with each one of the inner tubes extending through a respective one of the outer tubes. A plurality of perforated plates located in the shell are sealed releasably to respective ends of the inner and outer tubes thereby forming a plurality of liquid tight chambers for the flow of a plurality of segregated mediums and the transfer of heat therebetween.

Preferably, end portions of the plurality of outer and inner tubes are supported by and sealed releasably to the respective plates. The end portions project through bores in the plates and may be sealed to the plates by at least one circumferentially continuous gasket located in a counter bore of each one of the bores for radial compression between the plate and the end portion of the tube. Each one of the plates has a peripheral circumferential surface facing radially outward that preferably defines a continuous groove for seating a circumferentially continuous gasket that compresses radially between the continuous surface and an inward circumferential face of the shell.

Objects, features and advantages of the present invention include a heat exchanger capable of operating with a wide and versatile range of heat transfer profiles and that has a removable core for maintenance and easy replacement of individual tubes and other components. Other advantages include the omission of expensive manufacturing processes such as brazing, the ability to use non-corrosive, light weight and relatively inexpensive components such as plastic, a relatively simple and robust design and a long and useful life.

DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of this invention will be apparent from the following detailed description, appended claims, and accompanying drawings in which:

FIG. 1 is a perspective view of a tubular heat exchanger embodying the present invention and illustrated in a marine engine application;

FIG. 2 is a side view of the tubular heat exchanger;

FIG. 3 an end view of the tubular heat exchanger;

FIG. 4 is a perspective cross section of the tubular heat exchanger with components removed to show internal detail and generally taken along line 4-4 of FIG. 3;

FIG. 5 is a perspective cross section of the tubular heat exchanger with components removed to show internal detail and generally taken along line 5-5 of FIG. 3;

FIG. 6 is a perspective view of the tubular heat exchanger with a cylindrical shell removed to show internal detail;

FIG. 7 is a perspective view of a core of the tubular heat exchanger;

FIG. 8 is an enlarged partial and perspective cross section of the tubular heat exchanger with the same internal components removed and taken from circle 8 of FIG. 4;

FIG. 9 is an enlarged partial and perspective cross section of the tubular heat exchanger taken from circle 9 of FIG. 8, but without any internal components removed;

FIG. 10 is an enlarged partial and perspective cross section of the tubular heat exchanger taken from circle 10 of FIG. 8;

FIG. 11 is a cross section of an end cap of the heat exchanger;

FIG. 12 is an exploded perspective view of a perforated outward plate of the core of the heat exchanger;

FIG. 13 is a partial cross section of the outward plates; and

FIG. 14 is a partial cross section of a perforated inward plate of the core of the heat exchanger and similar in perspective to FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the following text sets forth a detailed description of numerous different embodiments of the invention, it should be understood that the legal scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only, and does not describe every possible embodiment of the invention because such would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the invention.

It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘______’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term be limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. § 112, sixth paragraph.

Referring now to FIG. 1 of the drawings, a tubular heat exchanger or cooler 20 embodying the present invention is illustrated as part of a preferred marine combustion engine assembly 22. The assembly 22 has a combustion engine 24, preferably two water cooled exhaust manifolds 26, a coolant pump 28, and an oil pump (not shown) preferably housed within an oil pan 30 of the engine 24. The heat exchanger 20 generally interacts with an open liquid coolant loop 32 that preferably flows lake or sea water, a closed coolant loop 34 that preferably flows a rust inhibiting coolant such as glycol for cooling the engine, and a closed oil loop 36 that removes heat and lubricates the engine 24. The heat exchanger 20 preferably functions to cool the engine oil via the engine glycol, and the engine glycol is cooled by the lake or seawater flowing through the open loop 32.

Preferably, a coolant pump 40 positioned in an inlet leg 38 of the open loop 32 flows coolant/water from an open body of seawater to the heat exchanger 20. The pump 40 may be mechanically powered by the operating engine 24, but preferably is operated by a variable speed electric motor and a control system (not shown) that senses various parameters such as oil temperature and glycol temperature and thereby adjusts the speed of pump 40 to optimize engine performance. An outlet leg 42 of the open loop 32 preferably tees-off to flow heated seawater through respective exhaust manifolds 26, known in the marine industry, before being expelled back into the open sea.

Referring to FIGS. 1-4, the heat exchanger 20 has a substantially cylindrical shell 46 extending longitudinally along a centerline 47 and between opposite ends 48, 50 where respective inlet and outlet nozzles 52, 54 are generally located for the flow of engine oil or lubricant delivered by the oil pump of the engine 24. Preferably, the ends 48, 50 of the shell 46 are circular openings and the nozzles 52, 54 are a unitary part of respective end caps 56, 58 that releasably seal to the cylindrical shell 46. Inlet and outlet ports 60, 62 are in the shell 46 and may be defined by radially outward projecting nipples 64, 66 of the shell 46 for coolant flow of the closed loop 34. Preferably, the nipples 64, 66 are spaced axially inward from adjacent respective ends 48, 50. Inlet and outlet orifices 68, 70 are in the shell 46 and may be defined by radially outward projecting nipples 72, 74 of the shell 46 for the coolant (seawater) flow of the open loop 32. Preferably, the nipples 72, 74 are spaced axially inward from adjacent respective nipples 64, 66 of the shell 46. With the centerline 47 being substantially horizontal, all of the nipples 64, 66, 72, 74 preferably project upward to prevent air from being trapped inside the heat exchanger 20. Each nipple 64, 66, 72, 74 preferable has a circumferentially continuous barb 76 projecting radially outward for snugly and sealably fitting to resiliently flexible hoses (not shown) of respective loops 32, 34.

Referring to FIGS. 4-7, a core 78 of the heat exchanger 20 fits sealably inside the shell 46 and is easily removable for cleaning, maintenance and/or replacement of internal components. This ability to inspect and maintain the core 60 is particularly advantageous in marine applications where the flow of salt water tends to be corrosive for some materials and the likelihood of marine growth and other deposits that reduce cooling efficiency is relatively high (e.g. barnacles). The removable core 78 of the heat exchanger 20 has a pair of perforated outward plates 80, 82 located axially inward from respective end caps 56, 58, a pair of perforated inward plates 84, 86 spaced axially inward from respective outward plates 80, 82 and a plurality of perforated diverter plates 88 spaced axially away from one-another and spaced axially inward from the inward plates 84, 86.

The heat exchanger 20 has inlet and outlet chambers 90, 92 defined axially between respective end caps 56, 58 and respective outward plates 80, 82 for the flow of oil, inlet and outlet chambers 94, 96 defined axially between respective outward plates 80, 82 and respective inward plates 84, 86 for the flow of glycol, and a mid chamber 98 defined axially between the inward plates 84, 86 for the flow of seawater that is generally diverted by the diverter plates 88 located in the mid chamber 98. The chambers 94, 96, 98 are also defined radially by an inward face 100 of the shell 46. The ports 60, 62 communicate directly with respective inlet and outlet chambers 90, 92, and the orifices 68, 70 both communicate with the mid chamber 98, but adjacent to respective inward plates 84, 86 with the diverter plates 88 positioned axially between the orifices 68, 70.

Referring to FIGS. 6-10, the core 78 of the heat exchanger 20 has a plurality of outer tubes 102 for the flow of coolant or glycol of the closed loop 34. Each tube 102 is substantially parallel to the centerline 47 and has a mid portion 104 that extends through the mid chamber 98. As best shown in FIG. 9, each tube 102 has opposite, distal, end portions 106 that project through respective inward plates 84, 86 and partially into respective inlet and outlet chambers 94, 96. Preferably, the mid portion 104 of each tube 102 has a diameter that is greater than a diameter of the end portions 106. This difference in diameters forms an annular stop 108 that faces and may abut respective inward plates 84, 86 for maintaining a predetermined distance between the plates 84, 86.

The core 78 has a plurality of inner tubes 110 for flow of oil of the closed loop 36. Each inner tube 110 extends through a respective outer tube 102 and through the mid chamber 98 and through the inward chambers 94, 96. Each tube 110 has opposite, distal, end portions 112 that project through respective outward plates 80, 82 and partially into respective inlet and outlet chambers 90, 92. Together, the outer and inner tubes 102, 110 define a channel 114 that has an annular cross section for the flow of coolant or glycol, and alone the inner tube 110 defines a channel 115 that has a round cross section preferably for the flow of oil.

Referring to FIGS. 8, 10 and 11, the end caps 56, 58 are preferably identical for reducing the required number of parts and reducing manufacturing costs. Each cap is preferably made of heat resistant, injection molded plastic. The respective nozzles 52, 54 are contoured to accept a fitting 116 for attachment to the oil loop 36. Preferably, the fitting 116 is of a threaded, metallic or brass, quick connect or compression variety. Each cap 56, 58 also has a circumferentially continuous collar 118 having an annular surface 119 that faces the respective outward plates 80, 82, a circumferential inner surface 120 that generally radially defines the inlet and outlet chambers 90, 92, and a circumferential outer surface 122 that faces and preferably is in contact with the inner face 100 of the shell 46. The outer surface 122 defines a circumferentially continuous groove 124 in the collar 116 for seating a resiliently compressible gasket or o-ring 126 for providing a liquid tight seal radially between the end caps 56, 58 and the shell 46.

Referring to FIGS. 10, 12 and 13, the outward plates 80, 82 are preferably identical for reducing the required number of parts and reducing manufacturing costs. Each outward plate 80, 82 is preferably made of injection molded plastic, and when assembled, is orientated substantially perpendicular to centerline 47. Each plate 80, 82 has an outer and an opposite inner face 128, 130. The outer face 128 defines in-part the chambers 90, 92 and the inner face 130 defines in-part the chambers 94, 96. Preferably, the periphery of the outer face 128 of the outward plates 80, 82 is in contact with the annular surface 119 of the collar 118 of respective end caps 56, 58 for axial spacing with respect to centerline 47. Similar to the end caps 56, 58, the outward plates 80, 82 each have a circular edge or peripheral surface 132 that faces radial outward and toward the inner face 100 of the shell 46. The peripheral surface 132 defines a circumferentially continuous groove 134 for seating a resiliently compressible gasket or o-ring 136. The o-ring 136 provides a liquid tight seal radially between the outward plates 80, 82 and the shell 46.

The two end portions 112 of each one of the inner tubes 110 extends through a bore 138 in the respective outward plates 80, 82. Each bore 138 communicates through both the outer and inner faces 128, 130 of the plates 80, 82, and has a counter bore 140 that communicates through the inner face 128 only. The diameter of the bore 138 at the outer face 128 is substantially equal to or slightly greater than the diameter of the end portions 112 of the inner tube 110. The counter bore 140 generally seats two resiliently flexible o-rings 142, 144, a rigid spacer ring 146 and a retainer 148. The first o-ring 142 is located at the bottom of the counter bore 140. The spacer ring 146 is located axially between the two o-rings 142, 144, and the retainer 148 is located axially between the o-ring 144 and inner face 130 and press fitted to the plates 80, 82. Both the spacer ring 146 and the retainer 148 are preferably made of plastic.

Referring to FIGS. 9 and 14, the inward plates 84, 86 are preferably identical for reducing the required number of parts and reducing manufacturing costs. Each inward plate 84, 86 is preferably made of injection molded plastic, and when assembled, is orientated substantially perpendicular to centerline 47. Each plate 84, 86 has an outer and an opposite inner face 150, 152. The outer face 150 defines in-part the chambers 94, 96 and the inner faces 152 axially define the mid chambers 98. Similar to the end caps 56, 58, the inward plates 84, 86 each have a circular edge or peripheral surface 154 that faces radial outward and toward the inner face 100 of the shell 46. The peripheral surface 154 defines a circumferentially continuous groove 156 for seating a resiliently compressible gasket or o-ring 158. The o-ring 158 provides a liquid tight seal radially between the inward plates 84, 86 and the shell 46.

The two end portions 106 of each one of the outer tubes 102 extends through a bore 160 in the respective outward plates 84, 86. Each bore 160 communicates through both the outer and inner faces 150, 152 of the plates 84, 86, and has a counter bore 162 that communicates through the inner face 152, but not the outer face 150. The diameter of the bore 160 at the outer face 152 is substantially equal to or slightly greater than the diameter of the end portions 106 of the outer tube 102. The counter bore 162 generally seats two resiliently flexible o-rings 164, 166, a rigid spacer ring 168 and a retainer 170. The first o-ring 164 is located at the bottom of the counter bore 162. The spacer ring 168 is located axially between the two o-rings 164, 166, and the retainer 170 is located axially between the o-ring 166 and inner face 152 and press fitted to the plates 84, 86. Both the spacer ring 168 and the retainer 170 are preferably made of plastic.

A spacer member or plurality of pins 174 of the core 78 extend axially between each one of the outward plates 80, 82 and the respective inward plates 84, 86 to maintain a predetermined axial distance with respect to centerline 47. A plurality of blind bores 172 (see FIGS. 8 and 9) in each one of the inward plates 84, 86 communicates through a peripheral portion of the outer face 150. Each one of a plurality of pins 174 has a first end 176 press fitted into a respective one of the bores 172. The pins 174 project axially outward to opposite distal ends 178 that contact or abut the inner face 130 of the outward plates 80, 82 for spacing the inward plates 84, 86 from the respective outward plates 80, 82 and thus maintaining a predetermined volume of the chambers 94, 96. As illustrated, there are preferably six pins 174 spaced circumferentially away from one another with respect to centerline 47 and projecting axially outward from each inward plate 84, 86. One skilled in the art, however, would now know that the pins could be made of any non-corrosive material having sufficient rigidity, or the pins and the inward plate could be molded as one unitary component. Moreover, the pins 174 could project from the outward plates 80, 82 as oppose to the inward plates 84, 86 and/or need not be pins at all but may take the form of any variety of shapes capable of spacing the plates away from one-another while not blocking coolant flow through the ports 60, 62.

Referring to FIGS. 4-7, the diverter plates 88 are perforated similarly to the inward plates 84, 86 for receipt of the outer tubes 102. Unlike plates 84, 86, the diverter plates are half circles or half of an otherwise circular disc having a curved edge that fits slideably against the inner face 100 and a straight edge that generally intersects the centerline 47. As previously described, the diverter plates 88 are in the mid-chamber 98 and are axially spaced from one another. Because adjacent plates 88 are also opposed to one another, flow of the seawater through the mid chamber 98 is diverted for improved cooling efficiency. The plates 88 also contribute toward structural rigidity of the core 78 and the outer tubes 102.

Preferably, the o-rings 126, 136, 158, 142, 144, 164, 166 are made of a resiliently flexible, rubber-like, material that is heat resistant such as viton. The inner and outer tubes 110, 102 are preferably made of metal having a high heat transfer coefficient and that is resistant to corrosion and marine growth such as copper or cupronickel. Preferably and for structural strength, the shell 46 is also made of copper or cupronickel. Unlike traditional heat exchangers, galvanic reaction concerns between various metal components is alleviated because of the plastic and o-ring components of the present heat exchanger as previously described. Moreover, any concerns with stress cracking is also alleviated because of the absence of brazing the tubes. The o-rings of the heat exchanger 20 not only provide the sealing function of traditional brazing but also permit thermal expansion and contraction between components made of different materials.

During assembly of the heat exchanger 20, the o-rings 164, 166, spacer ring 168 and retainer 170 are premounted in the counter bores 162 and the o-ring 158 is preferably preseated in the groove 152 of the inward plates 84, 86. Similarly, the o-rings 142, 144, spacer ring 146 and retainer 148 are premounted in the counter bores 140 and the o-ring 136 is preferably preseated in the groove 124 of the outward plates 80, 82. The ends 176 of the spacer member or pins 176 may then be press fitted into respective blind bores 172 in the plates 84, 86.

The mid portion 104 of the outer tubes 102 are then inserted through and generally seated to the diverter plates 88. The end portions 106 of the outer tubes 102 are then inserted through the bores 160 with the inner surfaces 152 of the plates 84, 86 opposing one-another. During this insertion, the o-rings 164, 166 resiliently compress about the end portions 106 for a liquid tight seal. This insertion preferably ceases when the annular stops 108 abut or come in near contact with the respective retainers 148.

The inner tubes 110 are then inserted through each respective one of the outer tubes 102 and the end portions 112 are of the inner tubes 110 are then inserted through the bores 138 with the inner surfaces 130 of the outward plates 80, 82 opposing one-another. During this insertion, the o-rings 142, 144 resiliently compress about the end portions 112 for a liquid tight seal. This insertion generally ceases when the distal ends 178 of the spacer pins 174 abut the inner surfaces 130 of the outward plates 80, 82.

With the core 78 preassembled, it may then be slid axially into the shell 46 from either of the openings defined by the ends 48, 50. Preferably, the ends 48, 50 are flared radially outward for easy insertion of the core 78. When the core 78 is axially moved into the shell 46, the o-rings 136, 158 are first cleared by either of the open ends 48, 50 and then compress against the inner face 100 of the shell 46. Because of the symmetrical design of the core 78, rotationally indexing the core 78 with respect to the centerline 47 and with respect to the shell 46 is not required. With the core 78 axially centered in the shell 46, the end caps 56, 58 with the preseated o-rings 126 are press fitted to the respective ends 48, 50. Contact of the annular surface 119 of the end caps 56, 58 with the outer face 128 of the respective outward plates 80, 82 assures that the core 78 is properly centered.

The angular position of the nozzles 52, 54 may then be independently adjusted by rotating the respective end caps 56, 58 about the centerline 47. Once adjusted, a plurality of threaded fasteners 180 are inserted through holes in the ends 48, 50 of the shell 46 and threaded into the end caps 56, 58. With the fasteners 180 engaged, the end caps 56, 58 are prevented from shifting axially. One skilled in the art, however, would now know that the end caps may releasably engage the shell 46 in a variety of ways. For instance, the caps 56, 58 may carry threads that threadably engage threads carried by the shell 46, thus fasteners would not be required. In this embodiment, the o-rings 126 can be compressed axially as oppose the illustrated radial compression. In yet another modification, the caps may be clamped to the shell 46.

During disassembly of the heat exchanger 20 for inspection and maintenance reasons, both end caps 56, 58 may be removed. With both ends of the shell 46 open, the core 78 can be pushed at one end and pulled out from the other. This technique is particularly advantageous if sediment build-up has occurred within the heat exchanger 20 that may otherwise make pulling of the core 78, from one end alone, difficult.

While the forms of the invention herein disclosed constitute a presently preferred embodiment, many others are possible. For instance, although the heat exchanger is generally described as a cooler, it could also function as a heater. Moreover, the liquid coolants (e.g. glycol and seawater) and oil may be any other form of a flowable medium and is not limited to liquids alone. Yet further, although the embodiment described entails three flowing mediums, the same novel aspects can be applied to heat exchangers having two, four or more flowing mediums. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention. 

1. A multi pass heat exchanger comprising: a tubular shell having an open first end and an opposite second end; a first end cap having an outlet for flow of a first medium; a second end cap having an inlet for flow of the first medium; a perforated first plate located in the shell and between the first and second end caps, wherein an outlet chamber is defined between the first end cap and the first plate for flow of the first medium; a perforated second plate located in the shell and between the first plate and the second end cap, wherein a second chamber is defined between the first and second plates for flow of a second medium; a perforated third plate located in the shell and between the second plate and the second end cap, wherein a mid chamber is defined between the second and third plates for flow of a third medium; a perforated fourth plate located in the shell and between the third plate and the second end cap, wherein a fourth chamber is defined between the third and fourth plates for flow of the second medium and a fifth chamber is defined between the fourth plate and the second end cap for flow of the first medium; a first tube communicating between the second and fourth chambers for flow of the second medium between the second and fourth chambers, the first tube having opposite end portions extending through and supported by the respective second and third plates; and a second tube communicating between the first and fifth chambers for flow of the first medium from the fifth chamber to the second chamber, the second tube having opposite end portions extending through and supported by the respective first and fourth plates.
 2. The multi pass heat exchanger set forth in claim 1 wherein the second tube coextends longitudinally with and is spaced radially inward from the first tube.
 3. The multi pass heat exchanger set forth in claim 2 further comprising an annular channel defined between the first and second tubes for flow of the second medium.
 4. The multi pass heat exchanger set forth in claim 1 further comprising: a centerline of the tubular shell; the first end cap, the first plate, the second plate, the third plate and the fourth plate each have a peripheral surface facing radially outward with respect to the centerline and defining a circumferentially continuous groove; and a resiliently flexible gasket located in each one of the grooves and sealed releasably between the respective peripheral surface and an inward face of the shell.
 5. The multi pass heat exchanger set forth in claim 4 wherein the gasket is an o-ring.
 6. The multi pass heat exchanger set forth in claim 1 further comprising a first spacer member extending between the first plate and the second plate and a second spacer member extending between the third plate and the fourth plate.
 7. The multi pass heat exchanger set forth in claim 1 wherein the opposite end portions of the first tube have a diameter that is less than a diameter of a mid portion of the first tube for spacing the second plate from the third plate.
 8. The multi pass heat exchanger set forth in claim 7 wherein the opposite end portions of the first tube have a diameter that is less than a diameter of a mid portion of the first tube for spacing the second plate from the third plate.
 9. The multi pass heat exchanger set forth in claim 1 further comprising: a centerline of the shell; a through bore in the first plate and through which the end portion of the second tube projects axially with respect to the centerline; a through bore in the fourth plate and through which the opposite end portion of the second tube projects axially; a first circumferentially continuous gasket seated in the through bore in the first plate and releasably compressed radially between the end portion of the second tube and the first plate; and a second circumferentially continuous gasket seated in the through bore in the fourth plate and releasably compressed radially between the opposite end portion of the second tube and the fourth plate.
 10. The multi pass heat exchanger set forth in claim 9 further comprising: a through bore in the second plate and through which the end portion of the first tube projects; a through bore in the third plate and through which the opposite end portion of the first tube projects; a first circumferentially continuous gasket seated in the through bore in the second plate and releasably compressed radially between the end portion of the first tube and the second plate; and a second circumferentially continuous gasket seated in the through bore in the third plate and releasably compressed radially between the opposite end portion of the first tube and the third plate.
 11. A heat exchanger comprising: an outer shell; a perforated first plate located in and attached sealably to the shell, the first plate having an inner face; a perforated second plate located in and attached sealably to the shell, the second plate having an inner face facing the inner face of the first plate; a mid chamber located between the first and second plates; a first bore in the first plate; a second bore in the second plate; a tube extending longitudinally through the mid chamber, the tube having a first end portion and an opposite second end portion projecting through the respective first and second bores; a circumferentially continuous first gasket for releasably sealing the first end to the first plate; and a circumferentially continuous second gasket for releasably sealing the second end to the second plate.
 12. The heat exchanger set forth in claim 11 further comprising a mid portion of the tube having a larger diameter than diameters of the first and second end portions for spacing the first and second plates apart.
 13. The heat exchanger set forth in claim 11 further comprising: first and second counter bores of the respective first and second bores; first and second retainers press fitted in the respective first and second bores; and wherein the first and second gaskets are located at the bottom of the respective first and second counter bores and the first and second retainers are located substantially flush with the inner face of the respective first and second plates.
 14. The heat exchanger set forth in claim 13 further comprising: a first spacer ring located adjacent to the first gasket in the first counter bore; a circumferentially continuous third gasket located between the first spacer ring and the first retainer in the first counter bore; a second spacer ring located adjacent to the second gasket in the second counter bore; and a circumferentially continuous fourth gasket located between the second spacer ring and the second retainer in the second counter bore.
 15. The heat exchanger set forth in claim 12 further comprising: first and second counter bores of the respective first and second bores; first and second retainers press fitted in the respective first and second bores; and wherein the first and second gaskets are located at the bottom of the respective first and second counter bores and the first and second retainers are located substantially flush with the inner face of the respective first and second plates; and opposite annular stops carried by the mid portion for abutting respective first and second retainers for maintaining a predetermined distance between the first and second plates.
 16. The heat exchanger set forth in claim 11 wherein the first and second plates are made of plastic and the tube is made of a copper alloy.
 17. The heat exchanger set forth in claim 16 wherein the first and second gaskets are o-rings.
 18. A tubular heat exchanger comprising: a cylindrical outer shell extending along a centerline and having a circumferential inner face, a first end, an opposite second end, an inlet orifice and an outlet orifice, wherein the first end defines a circular opening and the inlet and outlet orifices communicate radially through the shell; a first end cap engaged removably to the first end for sealing off the opening, the end cap having a first nozzle for flow of a first fluid; a second nozzle at the second end for flow of the first fluid, a removable core having a perforated first plate, a perforated second plate, and a plurality of first tubes extending longitudinally between and communicating through the first and second plates, the first and second plates each having an outer peripheral surface constructed and arranged to releasably seal to the inner face; a first end chamber defined axially between the first end cap and the first plate; an opposite second end chamber defined axially between the second end and the second plate; a mid chamber communicating with the first and second orifices and located axially between the first and second plates and defined radially by the inner face; wherein either one of the first and second nozzles flows the first fluid into the adjacent first or second chamber and the other of the first and second nozzles flows the first fluid out of the adjacent first or second chamber; and wherein a second fluid flows into the mid chamber via the inlet orifice and out of the mid chamber via the outlet orifice.
 19. The tubular heat exchanger set forth in claim 18 further comprising: the removable core having a perforated third plate located axially between the first end cap and the first plate in the shell, a perforated fourth plate located axially between the second end and the second plate in the shell, and a plurality of second tubes communicating through and supported at opposite end portions by the third and fourth plates; a fourth chamber defined axially between the first and third plates; a fifth chamber defined axially between the second and fourth plates; and first and second ports in the shell communicating radially with the respective fourth and fifth chambers for the flow of a third medium in and out of the shell.
 20. The tubular heat exchanger set forth in claim 19 wherein each one of the plurality of first tubes project co-axially through a respective one of the plurality of second tubes. 